Philosophical trend which emphasise the material world (the world outside of consciousness) as the foundation and determinant of thinking, especially in relation to the question of the origin of knowledge. For materialism, thoughts are “reflections” of matter, outside of Mind, which existed before and independently of thought.

Idealism is a thought process (ex. rationalism) of how the material world adheres to ideas. Idealists follow a certain ideal concept (ex. faith) and understand everything from its adherence to that concept.

Idealism can also be understood as the practice of understanding abstractions through other abstractions; where an abstraction is something that does not necessarily have basis nor relation to reality, but only exists in relation to other abstractions. The primary concern for the idealist is to create concepts that adequately explain (and change of viewpoint of) the world as we know it.

Idealism may reject the existence of the external world all altogether (the world beyond thought, beyond sensation) or assert that while a world beyond sensation may exist, it is unknowable. These trends are known as Subjective Idealism. On the other hand, idealism may accept the objectivity of nature but regard the material as the expression of ideal forces such as the Will of God, the absolute Idea, etc whose nature is accessible to the Mind directly. These trends are known as Objective Idealism.

https://www.marxists.org/archive/lenin/works/1913/mar/x01.htm

https://www.marxists.org/archive/marx/works/1845/german-ideology/ch01a.htm